Method for laser welding a workpiece

ABSTRACT

A method is used for laser welding a workpiece along a welding line. At least two laser beams, which are offset in relation to one another, are guided along the welding line by moving an optical deflector unit. The method may be used for welding closure pins and casting induced openings of gas turbine blades.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP01/15121 which has an Internationalfiling date of Dec. 20, 2001, which designated the United States ofAmerica and which claims priority on European Patent Application numberEP 00128575.8 filed Dec. 27, 2000, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a method for laser welding aworkpiece, in particular welding a closure pin into the casting-inducedopening of a cast gas turbine blade.

BACKGROUND OF THE INVENTION

In WO 00/19065 there is a description of a gas turbine blade and amethod for producing a gas turbine blade. In order to form such a castgas turbine blade through which coolant flows and which has passageopenings, at least one of which is induced by casting production, insuch a way that they can be produced with little scrap and largelyavoiding casting defects, it is proposed that one or more of the passageopenings induced by the casting process is or are passed through by arivet-like closure pin, the foot of which is secured on the oppositeside of an end wall.

To accomplish a high efficiency of the gas turbine, a working gastemperature that is as high as possible is required. The blades of thegas turbine, which are subjected to considerable loading as a result ofhigh or changing temperatures, pressures and centrifugal forces, aremetallic hollow bodies, the hollow spaces of which are flowed through bycoolant. The coolant flow proceeds from the root of the gas turbineblade, by which the latter is fastened to a rotor, through an innerhollow space with meandering air ducting chambers to outlet openings inthe airfoil region or in the pointed region of the gas turbine blade andforms a cooling film on the outer wall of the gas turbine blade.

To minimize the proportion of unused working gas flowing past the edgeregion of the gas turbine blades, the rotor with the inserted gasturbine blades is arranged inside the gas turbine in such a way that theenvelope of the tips of the gas turbine blades extends at the smallestpossible distance from the inner circumference of a peripheral, staticguide ring. In this respect it is necessary to ensure not only a closefit but also that the gas turbine blade is not damaged and the radiallyouter tip region is adequately cooled. In this tip region, the outerwall of the gas turbine blade on the tip side is therefore surrounded byprojecting edge pieces, the tip squealers. Cooling of this tip squealerregion takes place by means of a coolant flow which flows from the innerhollow spaces through passage openings in the end wall on the tipsquealer side into the tip squealer region and is led out again therethrough openings in the tip squealers, as disclosed in U.S. Pat. No.4,761,116.

A gas turbine blade with internal cooling and passage openings forcooling the tip squealer region constructed on a single level is knownfrom EP 0 340 149 B1. The production of such a gas turbine blade imposesincreased requirements on the construction of the casting molds and inparticular the core of the casting molds. To ensure consistent coolingof the tip squealer region, the thickness of the end wall on the tipsquealer side between the hollow space and the tip squealer region mustbe produced with close tolerances. The core part of the system of hollowspaces is rigidly connected to the core part of the tip squealer regionand in this way the two are kept at a fixed, constant distance from eachother during the casting process. This connection is established bysecuring pins, which are anchored in both core parts. The securing pinsare subjected to great loading during the casting process, as a resultof which they easily break. As a result, the securing pins are to beformed with a relatively great diameter.

However, this great diameter has the effect of producing large passageopenings in the end wall on the tip squealer side of the cast gasturbine blade between the hollow space and the tip squealer region.Consequently, the leakage of the coolant flowing through the passageopenings is very great. One possible way of reducing the leakage of thecoolant is to weld the large passage openings produced by the securingpins closed or to weld a cover plate onto them. This has thedisadvantage that it may have the effect of inducing cracks, whichincrease during the operation of the gas turbine blade. The hightemperatures during the welding may also cause recrystallizationprocesses, which weaken the material at the locations concerned, inparticular in the case of monocrystalline and directionally solidifiedgas turbine blades. The great loading caused by the centrifugal forcewhen the gas turbine blades are rotating leads more frequently to thepartial or complete detachment of the cover plate.

U.S. Pat. No. 3,761,201 describes a method for closing thecasting-induced openings in a gas turbine blade. A closure pin which hasa higher coefficient of thermal expansion than the surrounding materialis inserted into the casting-induced openings in an exactly fittingmanner. Diffusion welding is achieved by subsequent heating.

In the book “Wissensspeicher Lasertechnik” [compendium of lasertechnology] by Witlof Brunner and Klaus Junge, VEB FachbuchverlagLeipzig 1989, laser machining and welding is described on pages 291 to305. The construction of a laser material-machining installation isshown for example by FIG. 4.4 on page 291. A laser beam is directed in arigidly arranged laser beam guide onto a workpiece, which is arranged ona displaceable working bench. The guidance of the working being carriedout on the workpiece takes place by the displacement of the workpiece bymeans of the second working bench. The laser welding installationsdescribed on the subsequent pages also provide guidance for the workingby means of the movement of the workpiece. Welding with lasers isdiscussed on page 297.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for laserwelding a workpiece which makes it possible for the regions of materiallying around the weld to be impaired particularly little.

An object of the present invention is achieved according by providing amethod for laser welding a workpiece along a welding line in which alaser beam is guided along the welding line by movement of an opticaldeflecting unit.

It is consequently proposed for the first time to guide the laser beamduring the laser beam welding by way of the movement of an opticaldeflecting unit. While the workpiece has otherwise been moved under theworking laser beam, now the laser beam itself is moved. Keeping thismovement under control by an optical deflecting unit makes it possibleto accomplish this with comparatively little expenditure on apparatus.In particular, however, a high machining speed is achieved by themovement of such an optical deflecting unit. This has the consequencethat the heat input of the laser into the workpiece is locally greatlydelimited. As a result, the material adjacent to the weld is scarcelyaffected by the welding process. This counteracts in particular theformation of cracks caused by the welding process.

In one embodiment of the present invention, the welding line has arotational symmetry. It is further preferred for the welding line to becircular-symmetrical. In such an arrangement, it is particularlyfavorable to guide the laser beam in a rotational movement quickly alongthe welding line by means of the movement of the optical deflectingunit. It is further preferred for the welding line to have a maximumdiameter of 20 mm. Specifically in the case of such a comparativelysmall welding region, the movement of the laser beam requires a highangular speed, which can be accomplished by the movement of the opticaldeflecting unit. The fact that only the optical deflecting unit has tobe moved means that no further heavy parts, which can be moved only bycomplex structural design measures, of the associated laser installationneed to be moved.

In another embodiment of the present invention, the deflecting unit maybe rotated. It is further preferred in this case for the deflecting unitto have an optical axis, and also an axis of rotation oriented parallelto the optical axis, the axis of rotation being spaced apart from theoptical axis at an eccentric distance and the laser beam entering thedeflecting unit in parallel with and at a distance from the opticalaxis. A rotation of the deflecting unit can be accomplishedcomparatively easily in terms of apparatus. In particular, thedeflecting unit is a lens which is mounted eccentrically in a ballbearing and through which the laser beam passes eccentrically. Thiseccentric incidence on the one hand and the rotation of the lens on theother hand have the effect that the laser beam is deflected in acircular manner along the welding line. Correspondingly, it is alsopossible for example for a mirror to be used instead of the lens.Instead of a lens or a mirror, a prism or a wedge plate may of coursealso come into consideration for the deflecting unit. It is furtherpreferred for the eccentric distance to be variable during the weldingprocess. This also allows a welding line which is, for example,elliptical or spiral to be welded.

Furthermore, at the same time as the laser beam and offset in relationto it, a second laser beam is guided along the welding line by themovement of the optical deflecting unit. In particular, in the case of arotationally symmetrical welding line, the two laser beams may bearranged opposite each other, so that the movement of the opticaldeflecting unit respectively causes a half-arc of the welding line to bewelded by one of the laser beams. This has the consequence on the onehand of shortening the working time and on the other hand of making theheat distribution more uniform, which in turn favorably influences theimpairment of the material.

In a further embodiment of the present invention, the deflecting unit tobe a mirror which is tilted. This may be, for example, a planar mirror,which is arranged on a tripod arrangement of movable elements, which areadjusted in such a way that the mirror is tilted in a way correspondingto the desired direction of emergence for the working. In particular,piezoelectric adjusting elements are suitable.

In one embodiment of the present invention, the workpiece is a cast gasturbine blade, into which a closure pin is welded into a casting-inducedopening. It is further preferred for the gas turbine blade to the madeof a nickel or cobalt base superalloy. It is further preferred for it tobe directionally solidified or monocrystalline. As stated at thebeginning, in the case of a cast gas turbine blade, it is necessary forthe casting-induced openings to be closed to save cooling air. Aslikewise mentioned, in this case a conventional welding process leads toan impairment of the material, which applies in particular todirectionally solidified and monocrystalline gas turbine blades made ofa superalloy. The laser welding process described allows this impairmentof the material to be greatly reduced. However, for this it is necessaryto guide the laser beam comparatively quickly along the welding line.With the comparatively small dimensions of the openings to be closed,this leads to a high angular speed of the laser beam. This cannot beaccomplished by the movement of the workpiece or else of the laserinstallation, or only by very complex measures. This problem is solvedby movement just of the optical deflecting unit.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating exemplary embodiments of the presentinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 shows a plan view of a gas turbine blade;

FIG. 2 shows a detail of a gas turbine blade with a casting-inducedopening to be welded;

FIG. 3 shows a laser welding installation;

FIG. 4 shows a plan view of a welded workpiece;

FIG. 5 shows a plan view of a rotating deflecting unit;

FIG. 6 shows a longitudinal section through an optical deflecting unitwith a rotary drive;

FIG. 7 shows an optical deflecting unit formed with a tiltable mirror;and

FIG. 8 shows a plan view of a workpiece with welding by way of two laserbeams.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The same designations have the same meaning in the various figures.

FIG. 1 schematically shows the plan view of a gas turbine blade 1. Ablade airfoil 3 terminates with a blade tip 5. In this blade tip 5 thereare casting-induced openings 7. These have been produced by thesecurement of casting cores by means of securing pins, after removal ofthe securing pins following the casting. The gas turbine blade 1 isformed in a hollow manner in order to make it possible for cooling airto be introduced during operation for cooling the gas turbine blade 1. Aconsiderable part of this cooling air leaves through the casting-inducedopenings 7. To avoid this loss of cooling air, it is necessary to closethe casting-induced openings 7. A welding method which has particularlylittle effect on the material is laser welding, since the energy isintroduced in a targeted and directed manner (deep welding effect). Thispermits comparatively high working speeds with a moderate supply ofenergy.

FIG. 2 is a detail of a gas turbine blade 1 with a casting-inducedopening 7. A closure pin 11 has been inserted into the casting-inducedopening 7. This closure pin 11 is welded by means of a laser beam 13along a welding line 20 by producing a weld 15. So as not to affect thematerial surrounding the weld 15, it is necessary to guide the laserbeam 13 comparatively quickly along the welding line 20. Thecomparatively small dimensions of the casting-induced openings 7, with adiameter D of <30 mm, in particular <20 mm and preferably <10 mm, resultin a relatively high angular speed with which the laser beam 13 must bemoved.

FIG. 3 schematically shows a laser beam installation 14. Anoptical-fiber cable 16 serves the purpose of guiding the laser beam 13.By means of an optical system 18, the laser beam 13 is made to expandand run parallel. The laser beam 13 then enters an optical deflectingunit 17. This optical deflecting unit 17 has a lens 19, which isarranged rotatably about an axis of rotation 23. The optical axis 21 ofthe lens 19 is in this case oriented parallel to but offset from theaxis of rotation 23. The laser beam 13 enters the lens 19 along the axisof rotation 23. The rotation of the lens 19 has the effect that thelaser beam 13 is deflected in a circular manner. As a result, the laserbeam 13 is guided along the welding line 20 of the workpiece 1 to bewelded.

FIG. 4 shows a plan view of the workpiece of FIG. 3.

FIG. 5 shows a plan view of the optical deflecting unit 17. The opticalaxis 21 is arranged at an eccentric distance E from the axis of rotation23. By variation of the eccentric distance E during the welding process,welding lines 20 that are other than circular can also be welded. Inparticular, elliptical welding lines 20 or spiral welding lines 20 canbe achieved.

FIG. 6 shows in a longitudinal section the optical deflecting unit 17and its mounting and its drive. The lens 19 is secured in a ball bearing41. Arranged eccentrically in the ball bearing 41 is the lens 19, theeccentric distance E between the axis of rotation 23 of the ball bearing41 and the optical axis 21 of the lens 19 being adjustable. An electricmotor 43 and a V-belt 45 can be used to rotate the ball bearing 41, andconsequently the lens 19. Of course, it is also conceivable to use amirror or a prism instead of the lens 19.

FIG. 7 shows an optical deflecting unit 17, in which a mirror 51 istilted. For this purpose, the mirror 51 is arranged on adjustingelements 55 in the form of a tripod. These adjusting elements 55 may,for example, be piezoelectrically formed. Depending on the activation ofthe adjusting elements 55, the mirror 51 is tilted, in particular alsoin a rotationally symmetrical guide. The position of the mirror 51′ thatis represented by dashed lines produces the direction of emergence ofthe laser beam 13′, 61′ that is represented by a dashed line.

It is also possible for a number of laser beams to be guided by theoptical deflecting element 17. FIG. 8 shows this by the example of twolaser beams 13, 61. The two laser beams 13, 61 are arranged oppositeeach other in front of a circular welding line 20. Movement of theoptical deflecting unit 17 causes each of the laser beams 13, 61 todescribe a semicircle. This achieves the effect of shortening thewelding process on the one hand and a more uniform distribution of heatalong the welding line 20 on the other hand. To make the heatdistribution even more uniform and keep the cooling behavior undercontrol, two further laser beams 63 are also arranged opposite eachother and between the laser beams 13, 61. These further laser beams 63have a lower power and are preferably formed as diode lasers. They servefor keeping the cooling behavior along the welding line 20 under controland influencing it. As a result, a further reduction in the impairmentof the material and crack formation can be achieved.

Exemplary embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A method for laser welding a workpiece along a circular symmetricalwelding line, comprising: guiding a laser beam along the circularsymmetrical welding line by movement of an optical deflecting unit; andguiding concurrently a second laser beam along the circular symmetricalwelding line by the movement of the optical deflecting unit, wherein thelaser beam is offset in relation the second laser beam.
 2. The method asclaimed in claim 1, wherein the deflecting unit is a mirror which istilted.
 3. The method as claimed in claim 1, wherein the circularsymmetrical welding line has a rotational symmetry.
 4. The method asclaimed in claim 3, wherein the circular symmetrical welding line has amaximum diameter D of 20 mm.
 5. The method as claimed in claim 1,further comprising rotating the deflecting unit.
 6. The method asclaimed in claim 5, further comprising: rotating the deflecting unithaving an optical axis about an axis of rotation oriented parallel tothe optical axis, the axis of rotation being spaced apart from theoptical axis at an eccentric distance E; and allowing the laser beam tothe deflecting unit in parallel with and at a distance from the opticalaxis.
 7. The method as claimed in claim 6, wherein the eccentricdistance E is variable during the welding process.
 8. A method for laserwelding a workpiece along a welding line, comprising: guiding laser beamalong the welding line by movement of an optical deflecting unit; androtating the deflecting unit having an optical axis about an axis ofrotation oriented parallel to the optical axis, the axis of rotationbeing spaced apart from the optical axis at an eccentric distance E; andallowing the laser beam to enter the deflecting unit in parallel withand at a distance from the optical axis, wherein the eccentric distanceE is variable during the welding process.
 9. The method as claimed inclaim 8, further comprising guiding a second laser beam along thewelding line by the movement of the optical deflecting unit, whereinconcurrently the laser beam is offset from the second laser beam. 10.The method as claimed in claim 8, wherein the welding line has arotational symmetry.
 11. The method as claimed in claim 8, wherein theworkpiece is a cast gas turbine blade, into which a closure pin iswelded into a casting-induced opening.
 12. A method for laser welding aworkpiece along a circular symmetrical welding line, comprising: guidinga laser beam along the circular symmetrical welding line by movement ofan optical deflecting unit; and guiding concurrently a second laser beamalong the circular symmetrical welding line by the movement of theoptical deflecting unit, wherein the laser beam is offset in relationthe second laser beam, and wherein the workpiece is a cast gas turbineblade, into which a closure pin is welded into a casting-inducedopening.
 13. The method as claimed in claim 12, wherein the gas turbineblade includes a nickel or cobalt base superalloy.
 14. The method asclaimed in claim 13, wherein gas turbine blade is directionallysolidified or monocrystalline.
 15. A method for laser welding aworkpiece along a welding line, comprising: guiding a laser beam alongthe welding line by movement of an optical deflecting unit; and guidingconcurrently a second laser beam along the welding line by the movementof the optical deflecting unit, wherein the laser beam is offset inrelation the second laser beam, and wherein the workpiece is a cast gasturbine blade, into which a closure pin is welded into a casting-inducedopening.
 16. A method for laser welding a workpiece along a circularsymmetrical welding line, comprising: guiding a laser beam along thecircular symmetrical welding line by movement of an optical deflectingunit; and guiding concurrently a second laser beam along the circularsymmetrical welding line by the movement of the optical deflecting unit,wherein the laser beam is offset in relation the second laser beam, andwherein the laser beam and the second laser beam are concurrently guidedalong opposite sides of the circular symmetrical welding line.
 17. Amethod for laser welding a workpiece along a circular symmetricalwelding line, comprising: guiding a laser beam along the circularsymmetrical welding line by movement of an optical deflecting unit; andguiding concurrently a second laser beam along the circular symmetricalwelding line by the movement of the optical deflecting unit, wherein thelaser beam is offset in relation the second laser beam, and wherein eachof the laser beam and the second laser beam weld a semi-circular portionof the circular symmetrical welding line, the semi-circular portionsforming a circular portion of the circular symmetrical welding line. 18.A method for laser welding a workpiece along a circular symmetricalwelding line, comprising: guiding a laser beam along the circularsymmetrical welding line by movement of an optical deflecting unit;guiding concurrently a second laser beam along the circular symmetricalwelding line by the movement of the optical deflecting unit; andguiding, concurrently, a third laser beam and a fourth laser beam alongthe circular symmetrical welding line by movement of the opticaldeflecting unit; wherein the laser beam, the second laser beam, thethird laser beam, and the fourth laser beam are offset in relation toone another.
 19. The method of claim 18, wherein the laser beam and thesecond laser beam are guided along opposite sides of the circularsymmetrical welding line, and the third laser beam and the fourth laserbeam are guided along opposite sides of the circular symmetrical weldingline.
 20. A method for laser welding a workpiece along a circularsymmetrical welding line, comprising: guiding a laser beam along thecircular symmetrical welding line by movement of an optical deflectingunit; and guiding concurrently a second laser beam along the circularsymmetrical welding line by the movement of the optical deflecting unit,wherein the laser beam is offset in relation the second laser beam,wherein the laser beam and the second laser beam are concurrently guidedalong opposite sides of the circular symmetrical welding line, andwherein each of the laser beam and the second laser beam weld asemi-circular portion of the circular symmetrical welding line.